Rear Main Cap Oil Drain Pipe Dimensions
 

Does anyone have the spec on the oil drain pipe for the rear main? I am suspicious that mine is not the correct length (too short). I think that when the oil level lowers it is above the oil causing a leak, even when the engine is running.

Thanks

Dick

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

It may not matter. IIRC, the tube is "open" at the top to the scupper built into the lower bearing cap. Oil returning to the crankcase simply follows the tube down and prevents oil curling over the top of the scupper and following the inboard end of the cap to the square gasket and possible leak path past the square gasket to the outside.

The tube/scupper arrangement simply prevents a continual flow of oil over the square gasket which ain't all that great a seal in the first place relying on the knife edge of the oil pan for seal.

Joe K

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

The pipe on my spare cap is 4" long. The pipe should always exit in the oil so the crankcase pressure doesn't go up the pipe and help push oil out.

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

Dick, the length of main cap pipe makes all the difference in the world.

It is 4 inches long, counting the threads.

It should be a 3/8's hole in the pipe and cap, if it is a 5/16's, bore the cap out, and pipe hole, and thread again.

If the pipe is not the right length, crankcase pressure will push out oil back through the pipe, and out the rear main, or they wouldn't have even put a pipe in.

The cork that seals the rear main cap is a fool proof seal if done right, as if it leaks, you done it wrong!

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

The mechanics (and hydraulics) of that action eludes me entirely. If your crankcase is properly vented by a breather, how can pressure build up?

And were this the case wouldn't the annular ring of oil contained by the bearing blow out too?

In fact it would blow out before the tube since it doesn't have any hydraulic "head" to maintain it?

In fact, giving this thought, wouldn't a pressurized crankcase bad enough to require the tube to prevent a rear main leak also back up oil from the bearing supply in the valve chamber and prevent it's drop by gravity down to the mains?

There are some misconceptions and disagreements regarding the Model A engine. The rear main leak issue is certainly one.

Joe K

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

The crankcase is pressurized by cylinder blowby, no matter the condition of the rings, there is some blow-by. Putting the end of the drain tube in the oil isolates the tube from the blow-by pressure. It is at the atmospheric pressure at the rear of the bearing that is open to the air. If there were enough crankcase pressure, oil could be forced up the tube, but I have never heard of that. The amount of pressure needed to raise the oil up and out through the tube could be calculated.
The oil in the bearing can be and is pressurized from the crankcase to some extent, but that pressure has to overcome the resistance of clinging to the bearing, and moves slowly. Also, when oil in the bearing migrates rearward to the open area and galley at the rear of the bearing, where it is collected, it is at atmospheric pressure and drains down the tube by gravity. There is not much oil that gets back there if the bearing is assembled property and the galley area is able to collect it and direct it to the tube, into the pan.

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

Undoubtedly some blow by, but that is the point of the crankcase breather - to prevent the accumulation of pressure.

The rest of the paragraph supports my conclusion I think - although you may come to a different conclusion.

The original SEARS engine (painted red) in my 29 Truck was obviously the second engine and with 40K on the odometer one could conclude that this was actually 140K. The engine was severely worn out to the tune of 12 thousandths out of round for the rod journals. (It had a most unique "whuppa-whuppa" sound when in operation from the rods moving at right angles to the rod journals) The best adjustment I could make was to file the caps (ugh!) and bring the rods to a "pinching" condition on the rod journals. One more turn on each cap nut and the crank "froze" on the high spot of the egg shape. It was a losing battle on an overworn engine.

That engine had severe blow by. It used one of the "conduit" type breather caps which would route the gasses to the intake of the carburetor. Even so, everything was covered with oil/dirt and down the carburetor side of the engine.

And it had NO tube on the rear main. Oil was free to drip down from where the tube used to be, across the square gasket, down the inside of the pan.

And still no rear main leak.

But despite no tube and this flow path I may have been fortunate to have an effective seal on the square gasket - it happens regularly according to some.

Still, if you think of it - at the inboard side of the bearing where the inner scupper is, there is an annular gap - this gap permits the inside of the bearing cap to be exposed to crankcase pressure. Meanwhile, a connector gallery runs from the scupper AROUND the rear main cap (through actually) and connects to a similar "throw chamber" for the slinger ring. Low spot of this connector gallery connects to the pipe leading down to the crankcase oil level. This drain tube normally sealed by protruding below the oil surface. Thus BOTH sides of the rear main bearing are at crankcase pressure as is the TOP of the drain tube.

Surrounding the crankshaft back of the slinger ring throw chamber is another annular gap where oil could (finally) exit the bearing. Any pressure here will blow oil behind the flywheel.

Venting of the slinger ring area by the drain tube were it made short of the oil surface should help RELIEVE the slinger ring area. Not impede oil, although oil as a liquid should seek it's own level anyway.

Thus I hold that the length of the tube is immaterial PROVIDED it conducts oil away and across the square gasket. Pressure in the crankcase is negligible even on severely worn engines and any "head pressure" in the drain tube is not of consideration in the design.

Well, there I've said it out by the numbers. But opinions vary - and always will.

Anyway, I hope the discussion has intrigued.

Joe K

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

Herm is right!!!! A few years ago while driving from Va to Pa I stop at a rest stop to take a break, came back to car and oil had ran out under car,drove it on to Pa.,had to stop every 50 mi and put qt oil in,when I got there borrowed a jack and stands ,pulled oil pan off and oil tube was broke,put in tube and fixed it,I know it will blow oil out if the tube is broke!

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

There is another way to look at it also. The oil pump is drawing the oil to the pump at a fast rate. So maybe with that pipe in the oil, it is causing a vacuum in the tube to help the oil run faster from the rear main drain. Any way Ford did not use a tube for no reason.

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

PC, you don't have to push the oil up and out the tube, all you have to have happen is for the oil in the tube to stop flowing. Once the flow is stopped by the crankcase pressure caused by the up and down movement of the pistons, the oil returning through the rear main oil tube will overwhelm the slinger trough and run out the rear into the flywheel housing and out the cotter pin hole. I have personally had a tube fall off in my car and the leak was major and embarrassing to say the least. I have also rebuilt several engines with modified tubes that were too short that leaked quite badly.

Randall and George are right, especially George and his assessment of the pump's role in this. If you don't think the tube length matters, just cut one off 1" short and get ready to carry a gallon of oil with you on even short runs.

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

I agree. It has to do with the surface tension out of the main. With the tube, it is submersed in the oil with no surface tension thus allowing the oil to flow back into the pan. He was lucky that he didn't get the major leak without the tube.

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

But, but. The oil is in a reservoir. There is not physical connection between the oil pump and the drain tube. And PLENTY of oil to choose from besides what issues from the tube.

Regarding Randall's comment, Les Andrews relates:

THIS I can see. Given a serious enough slope, it is possible for the oil to flow to the back of the crankcase and cover (nearly) the annular gaps mentioned earlier. A check valve is a logical, but probably less than reliable solution to limit flow backwards to the rear main. (One imagines all that crap going back into the check and hanging it up!)

But I don't know Randall's specifics - nor any other changes he might have made to correct his leaking including repositioning/renewing that elusive to seal square cork gasket?

Even a minor change of position of the bearing cap can make a difference - even the dabs of permatex commonly used at the cork/oil pan gasket juncture are subject to variation?

I don't pretend to know all the answers. I'm not even sure I can ask all the questions! But to me the mechanics of the rear seal seem awfully "iffy" to attribute rear main leak to the tube, either alone or in combination with other aspects.

And I have seen NO tube work.

Joe K

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

There doesn't have to be a physical connection. It will draw oil to it simply because it flows so much in the first place. This is why loose rear mains don't leak much when the engine is running but cover the floor when stopped, no suction.

As for the check ball and slope leaks. You don't have to have a serious grade to cause a leak. I have parked cars on as little as a 7% grade and had a serious leak. 7% is almost anywhere you want to park in the western NC hills.

I'll send you a pan gasket and you can cut your tube 1" or just take it out and try it. I bet it leaks.

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

"This is why loose rear mains don't leak much when the engine is running but cover the floor when stopped, no suction."

Interesting point. I have had A engines through the years that would NOT leak a drop of oil while the engine was running but would leak about a half cup of oil when the engine was shut down.

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

Richard, it is a natural circulation created by the pump and it's super flow. I have had the pump to pump the oil low enough to cause the pressure gauge to drop to 0 when the oil is cold and won't return as fast as the pump will flow it.

As an aside, I went through the battery cables on the Cabbie and found a bad ground which I fixed. The car now spins over great and starts on the second stroke. Even the horn works better.

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

You mean that half a cup of oil that exists always in the valve chamber and when the engine stops and the crank moves to the rear and releases the thrust bearing then floods (and overflows) the slinger chamber?

As I have said - the rear main bearing is the juncture of MANY maps - and a tough place to fight a seal battle.

Hard to attribute surrender to any single battle skirmish.

Joe K

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

Joe, I have no ideal where the oil would leak from internally, only that when the engine was shut down about a half cup of oil (sometimes less with different engines) would drain out through the bell housing/back of oil pan area when the engine was shut down, usually overnight. I never thought that a little oil leak was that big a deal as the engines all ran great.

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

All I did was put a new tube in,used the same cork and same gasket,mine is not the only one I have seen break and leak like a oil well! Also I think the crankcase pressure is going to make it's way out every nook and cranny it can find.

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

You are right Randall.

Even a rebuilt motor, we always pour 1 gallon, would only need maybe 2 quarts, down the Dist. hole and prime all the oil tubes. Over night that will leak about 2 table spoons on the floor.

If you have to replace any oil tubes, even the one that came out of the cap, the soft plug should always be pulled to replace the pipe, as where ever the pipe stops in the hole, the excess should be cleaned out to the Dia. of the hole, and to check for any other debris.

We also tack, wire weld the pipe to the cap on both sides.

The more clearance you have, has to run out faster, as there is nothing to stop the free flow of the bearing fed oil pipe, and the cap pipe is in the oil and would have somewhat of back pressure compared to the feed pipe, at rest.

On the rear main cap cork that goes around the cap in the groove, some are coming now that are made wrong.

The cork should NOT be a perfect fit in the groove, they do not want to stay put. They should be about .50 thousandths wider then the groove, like the Original, to lazy today to go measure one.


The pan gasket should be put on the BLOCK first with the Black Permatex, and yes, even under the car.

Cut the main cork to size so it fits

Use Black Indian Head Permatex and coat the groove.

With the wide cork, put the side away from you in the groove first, and with a screw driver, medium size short, and push the other side of the gasket in, and it will wedge itself in place, and will stay.

Take care of the front seal area, or check it.

Put a Generous coat of Black Permatex on the pan gasket, and on the Rear main cap cork, and the pan will not leak.

Re: Rear Main Cap Oil Drain Pipe Dimensions
 

Every word is right. I don't like the indian head because of cleanup next time but, it will stop leaks. As for a gallon down the dist hole, I do the same because what is overfull in the valve chamber will go down the return tube to the dipper tray. All these are good practices.